The present invention relates to a switching power supply for supplying a stabilized DC voltage to industrial and consumer electronic appliances. More particularly, the present invention relates to improvements in the control stability of the DC-DC converter of a switching power supply.
In recent years, as electronic appliances are made more inexpensive, compact, efficient and energy saving, switching power supplies having high output stability and being compact and efficient are demanded strongly. In particular, in the case of power supplies for supplying electric power to semiconductor devices, as semiconductor devices are made more highly integrated, power supplies having higher stability at a lower voltage and capable of supplying a larger current are demanded strongly.
FIGS. 11A, 11B, 11C and 11D are circuit diagrams showing various configuration examples of DC-DC converters in conventional switching power supplies. FIG. 11A shows a forward-type DC-DC converter, FIG. 11B shows a flyback-type DC-DC converter, FIG. 11C shows a half-bridge-type DC-DC converter, and FIG. 11D shows a full-bridge-type DC-DC converter. In the forward-type DC-DC converter shown in FIG. 11A, numeral 100 designates an input DC power supply, and the series circuit of the primary winding 102a of a transformer 102 and a main switching device 101 is connected across the input DC power supply 100. The series circuit of rectifier diodes 103 and 104 is connected across the secondary winding 102b of the transformer 102, and one end of a rectifier choke coil 105 is connected to the connection point of the two rectifier diodes 103 and 104. The other end of the rectifier choke coil 105 is connected to one end of a smoothing capacitor 106. Both ends of the smoothing capacitor 106 are used as output terminals, and a load 107 is connected across the terminals. The output voltage supplied to the load 107 is detected by an output voltage detection circuit 111 and output to an error amplifier 109. The error amplifier 109 compares the output voltage with a reference voltage from an output voltage setting reference power supply 110, amplifies an error signal therebetween and outputs the error signal to a control circuit 108. The control circuit 108 carries out the ON/OFF control of the main switching device 101 on the basis of the error signal.
In the flyback-type DC-DC converter shown in FIG. 11B, components having the same functions and configurations as those of the forward-type DC-DC converter shown in FIG. 11A are designated by the same numerals. The flyback-type DC-DC converter shown in FIG. 11B has a configuration wherein the rectifier diode 104 and the rectifier choke coil 105 in the forward-type DC-DC converter shown in FIG. 11A are eliminated.
In the half-bridge-type DC-DC converter shown in FIG. 11C, numeral 120 designates an input DC power supply, and the series circuit of two capacitors 121 and 122 is connected across the input DC power supply 120. In addition, the series circuit of two main switching devices 123 and 124 is connected in parallel with the series circuit of the capacitors 121 and 122. In this case, MOSFETs are taken as examples of the main switching devices 123 and 124. The primary winding 125a of a transformer 125 is connected between the connection point of the two capacitors 121 and 122 and the connection point of the two main switching devices 123 and 124. The transformer 125 has a first secondary winding 125b and a second secondary winding 125c, and these windings are connected to rectifier switching devices 126 and 127, respectively. One end of an output choke coil 128 is connected to the connection point of the first secondary winding 125b and the second secondary winding 125c, and the other end of the output choke coil 128 is connected to a smoothing capacitor 129. Both ends of the smoothing capacitor 129 are used as output terminals, and a load 130 is connected across the ends. The output voltage supplied to the load 130 is detected by an output voltage detection circuit 134 and output to an error amplifier 132. The error amplifier 132 compares the output voltage with a reference voltage from an output voltage setting reference power supply 133, amplifies an error signal therebetween and outputs the error signal to a control circuit 131. The control circuit 131 carries out the ON/OFF control of the main switching devices 123 and 124 on the basis of the error signal.
In the full-bridge-type DC-DC converter shown in FIG. 11D, components having the same functions and configurations as those of the half-bridge-type DC-DC converter shown in FIG. 11C are designated by the same numerals. In the full-bridge-type DC-DC converter shown in FIG. 11D, instead of the capacitors 121 and 122 in the half-bridge-type DC-DC converter shown in FIG. 11C, main switching devices 135 and 136 that are ON/OFF controlled by a control circuit 131 are provided. By the ON/OFF operation of the main switching devices 135 and 136, a high-frequency voltage is generated in the primary winding 125a of a transformer 125. Hence, a high-frequency voltage corresponding to the turns ratio of the primary winding 125a and a secondary winding 125b is generated in the secondary winding 125b, and a high-frequency voltage corresponding to the turns ratio of the primary winding 125a and a secondary winding 125c is generated in the secondary winding 125c. In the full-bridge-type DC-DC converter shown in FIG. 11D, the voltages are rectified by rectifier switching devices 126 and 127 and smoothened by an output smoothing circuit comprising a reactor 128 and a capacitor 129 to obtain a DC voltage, and the DC voltage is supplied to a load 130. In addition, the output DC voltage is detected by an output voltage detection circuit 134 and input to one of the input terminals of an error amplifier 132. The reference voltage from a reference power supply 133 is input to the other input terminal of the error amplifier 132, and the output DC voltage is compared with the reference voltage by the error amplifier 132. A PWM pulse signal depending on the result of the comparison is supplied from the control circuit 131 serving as a pulse generator to the main switching devices, whereby the main switching devices are turned ON/OFF. As a result, a stabilized DC voltage is supplied to the load 130.
In conventional switching power supplies, a method of connecting a plurality of switching power supplies in series is used to make the capacity larger or to make the circuit components smaller and lighter. More specifically, power supplies for operating semiconductor devices are designed to supply a lower voltage and a larger current while the pattern lines of semiconductor devices, such as large-scale integrated circuits (LSI) and microprocessors (MPU), are made finer. In particular, in the case when the voltage step-down ratio between the input and output voltages is large, for example, in the case when the input voltage is 48 V and the output voltage is 1.2 V, the turns ratio of the transformer increases inevitably. In addition, in a switching power supply having this kind of configuration, when the amount of the output current increases, the losses in the windings of the transformer also increase. This results in making the switching power supply lower in efficiency and larger in size.
In the case when DC-DC converters serving as a plurality of switching power supplies are connected in series, an input DC voltage is divided by a plurality of capacitors. The-divided voltages are used as power supplies, and the DC-DC converters are respectively connected to the power supplies. These DC-DC converters are ON/OFF controlled by PWM signals supplied from a control circuit, whereby a desired DC voltage is generated on the output sides connected in parallel. Conventional examples comprising a plurality of switching power supply circuits connected in series are shown in FIGS. 12A and 12B. FIGS. 12A and 12B show circuit examples of conventional switching power supplies wherein DC-DC converters serving as a plurality of switching power supply circuits are connected in series on the input sides thereof. The circuit diagram shown in FIG. 12A is a schematic diagram showing a switching power supply formed of half-bridge-type or full-bridge-type DC-DC converters. The circuit diagram shown in FIG. 12B is a schematic diagram showing switching a power supply formed of forward-type or flyback-type DC-DC converters.
In FIGS. 12A and 12B, an input DC voltage of an input DC power supply 201 is divided by two voltage-dividing capacitors 202 and 203, and the divided DC voltages are input to two DC-DC converters 204 and 205, respectively. The outputs from the DC-DC converters 204 and 205 are smoothened by a smoothing capacitor 206 and supplied to a load 207. As described above, the input DC voltage is divided by the capacitors 202 and 203, and the divided voltages, used as power supplies, are input to the DC-DC converters 204 and 205, respectively. The DC-DC converters, the output sides of which are connected in parallel, are configured so as to supply a desired output voltage to the load 207.
However, in the conventional switching power supplies configured as shown in FIGS. 12A and 12B, deviations occur in the circuit constants and in the operations of the main switching devices in reality. As a result, in each of the DC-DC converters, the balance of the input voltages of the DC-DC converters is lost, and a load imbalance occurs in some cases. If this kind of load imbalance becomes large, the usage conditions of the main switching devices are adversely affected, and the switching power supply cannot carry out its function in some cases. Therefore, an object to be attained in this field is to provide a switching power supply capable of carrying out stable operation without causing the above-mentioned problems.
As switching power supplies developed to attain such an object, the switching power supplies disclosed in Official Gazette of Japanese Unexamined Patent Publication No. Sho 62-138061 and Official Gazette of Japanese Examined Patent Publication No. Hei 4-1589 are available.
Official Gazette of Japanese Unexamined Patent Publication No. Sho 62-138061 has disclosed a switching regulator power supply wherein the input sides of a plurality of high-frequency inverter circuits are connected in series. In this switching regulator power supply, applied voltages are balanced to prevent two switching devices from breaking. For this purpose, in the switching power supply, each of the high-frequency inverter circuits is operated at an opposite phase, and the choke coil on the output side is used in common, whereby the difference in impedance between the high-frequency inverter circuits is eliminated.
In addition, Official Gazette of Japanese Examined Patent Publication No. Hei 4-1589 has disclosed a method wherein the voltage target value to be shared by each capacitor connected to the input side of each of a plurality of DC-DC converters is determined, the voltage applied to each capacitor is detected, and control is carried out so that the deviation between the two voltage values becomes zero.
However, in the switching power supply disclosed in Official Gazette of Japanese Unexamined Patent Publication No. Sho 62-138061, if the constants of the components in the high-frequency inverter circuits other than the choke coils have variations, a difference occurs in load sharing. As a result, there is a fear of making its operation as a power supply unstable.
Furthermore, in the switching regulator power supply disclosed in Official Gazette of Japanese Unexamined Patent Publication No. Sho 62-138061, to which a high voltage is input, the voltages of each high-frequency inverter circuit are balanced so that the semiconductor devices constituting the high-frequency inverter circuits serving as switching devices are prevented from being broken by the application of voltages exceeding the withstand voltages of the devices. However, in the case of the configuration disclosed in Official Gazette of Japanese Unexamined Patent Publication No. Sho 62-138061, the input voltage is required to be divided by resistors when the voltage target value is set. Hence, a power loss occurs at the time of voltage detection, and efficiency is lowered.
In the series operation system for the DC-DC converters disclosed in Official Gazette of Japanese Examined Patent Publication No. Hei. 4-1589, the target value is adjusted so as to be a voltage value to be shared by each capacitor. However, since the components of the DC-DC converters have variations in their characteristics, the output states of the circuits are unbalanced.